This pushbutton toggles the window style ring. A LMB click changes to the next window style in a 'ring' of 5 styles. A RMB click reverses the direction of the ring and switches to the previous style.
Accelerator Keys - F11/F12
Note: There is a command available in the toolbar to toggle between the last and current styles. It is not in the default toolbar and must be added by the user. See configure toolbar and Adding a last window style toggle.